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According to the National Institute of Mental Health, ADHD occurs in an estimated 3 to 5 percent of preschool and school-age children. Therefore, in a class of 25 to 30 children, it is likely that at least one student will have this condition.

No one knows exactly what causes ADHD. There appears to be a combination of causes, from genetics to possible environmental influences. Physical differences in the brain seem to be involved. Several different factors could increase a child’s likelihood of having the disorder.

Gender. Although girls can have ADHD, boys are at much higher risk. One study found that about 2 to 3 times more boys than girls have ADHD.1

Family history. Having a biological parent or sibling with ADHD seems to raise a child’s chances of developing it. According to the National Institute of Mental Health, at least one-third of fathers who had ADHD in their youth may have a child with ADHD.

Prenatal risks. Some studies have shown a possible increase in risk among children whose mothers used cigarettes or alcohol during pregnancy. (If you are pregnant, do not smoke or use alcohol!)

Environmental toxins. Exposure to very high levels of lead before age 6 or so might also raise a child’s risk. Some young children are exposed to lead from the dust of worn paint in many older buildings, or from drinking water that has traveled through lead pipes.

ADHD begins in childhood, but it often lasts into adulthood. Several studies done in recent years estimate that 30 to 65 percent of children with ADHD continue to have symptoms into adolescence and adulthood.